In laser material processing, a processing laser beam is guided along a processing beam path through an optical arrangement consisting of lenses and/or mirrors in a laser processing head and focused onto the workpiece to be machined with a focusing device. For the purposes of observing the process during the laser material processing, a beam splitter (e.g. a wavelength-selective mirror or a scraper mirror) is conventionally arranged in the processing beam path within the laser processing head, where the beam splitter separates the observation beam path from the processing beam path.
By way of example, such a laser processing head is disclosed in DE10120251B4. The laser processing head described therein has a sensor apparatus that includes a spatially resolving receiver arrangement having imaging optics, a stop and a radiation-sensitive receiver. The imaging optics image an area in the region of the interaction zone between the laser beam and the workpiece on the stop, which is on the detector. For the purposes of selecting the detected observation field the stop may be displaceable in at least one direction perpendicular to the optical axis of the imaging optics.
A laser processing head for process observation, in which beams originating from the workpiece are fed to an observation channel by way of a deflection mirror arranged laterally in relation to the processing laser beam, has been disclosed in DE19630437 A1.
In the case of coaxial process observation, the observation beam path passes through the focusing device, often with the beam axis of the processing laser beam coinciding with the optical axis of the observation beam path. Typically, the focusing device has a focusing lens, or consists of a focusing lens, that focuses the processing laser beam onto the workpiece but can result in a number of problems. For example, laser radiation with high power passes through the focusing lens when processing and a thermal lens forms as a result of the absorption of the processing laser radiation in the lens substrate and in the lens coating that is typically present. That is, the refractive index of the substrate is no longer homogeneous but changes with the forming temperature gradient. In the ideal case of a rotationally symmetric distribution of the power of the processing laser beam in the focusing lens, the focusing lens has the greatest temperature at the center and the temperature reduces radially to the outside toward the edge of the lens. If using a CO2 processing laser beam and a zinc selenide lens for focusing the processing laser beam, significant focal length shortenings of the focusing lens of up to 5% occur with increasing laser power and processing duration. If using solid state lasers and quartz optics as a beam source for producing the laser beam, the effect is less pronounced but likewise occurs in the case of large power densities on the focusing lens.